Let us continue our examination of image circle and sensor size. As we determined, using a projector that produces a larger image circle than the screen has some difficulties. You get a cropped in view of the presentation and record less resolution than the projector creates. What about if the opposite situation happens?

You walk into the room and turn on the projector and the screen is big enough. In fact it’s too big. The image circle fits within the screen and there’s room on the side. When photographers talk about this situation it’s called the image circle issue. The first scenario we discussed is simple called the crop factor.

What does this mean? There’s blank area on the screen that has no image on it. When you put a lens onto a camera whose image circle is smaller than the sensor (an APS-C lens on a full frame camera, for instance) there is a black space on the edges and you see the image within a circle. Different companies will combat this concern in different ways. Canon doesn’t even let their APS-C lenses lock on to their full frame bodies. Nikon and Sony will, but then they need to compensate.

The way these manufacturers address these consequences is to lose resolution. The camera identifies the image circle within the sensor and then creates an image inside it. This is shown below.

While an APS-C lens on a full frame sensor uses all the lens’s glass, it can only utilize less than half the pixels on the sensor. For instance, if you shoot with a 24 million pixel sensor and use an APS-C lens, you will record about a 10MP image. So if we use a full frame lens on an APS-C sensor, we use all the camera’s resolution but only half the lens’. If we use a full frame lens on an APS-C camera, we use all the lens’s resolution, but less than half of the camera’s. Does this mean we should only ever match a lens to a sensor?

Not necessarily. There are factors at least as important as full employment of resolution. Many photographers use full frame lenses on APS-C bodies and are very happy. Sometimes shooting with an APS-C camera gives them extra zoom that is more important to their style of shooting (namely sports) than full use of resolving power would be. And after all, just matching image circle to sensor may not be the key to getting the background and depth of field that you’re looking for. Next week we will examine angle of view to prove this point. Stay tuned. In the meantime, here is a picture of a screech owl.